Retroreflective Sheet Structure

ABSTRACT

A retroreflective sheet structure ( 10 ) comprising a transparent layer ( 20 ) having a front light-receiving surface ( 30 ) and a rear retroreflecting surface ( 32 ). Light incident on the front surface ( 30 ) will pass through the layer ( 20 ), impinge on the rear retroreflective surface ( 32 ) and reflect back out through the front surface ( 30 ) in a predetermined direction. An identifying indicia ( 44 ) is chosen and then formed on the retroreflecting surface ( 32 ). This indicia ( 44 ) can be used for identification purposes, even years after an end product incorporating the reflective sheet structure ( 10 ) has been out in the field.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a division of U.S. application Ser. No.11/915,930 filed Nov. 26, 2007, which is a national phase application ofInternational Application No. PCT/US2006/022173 filed Jun. 7, 2006,which claims priority to U.S. Provisional Patent Application No.60/691,338 filed Jun. 16, 2005 which are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

This invention relates generally, as indicated, to a retroreflectivesheet structure and, more particularly, to a retroreflective sheetstructure comprising a transparent thermoplastic layer with a frontlight-receiving surface and a rear retroreflecting surface.

BACKGROUND OF THE INVENTION

A retroreflective sheet structure comprises a transparent thermoplasticlayer having a front light-receiving surface and a rear retroreflectingsurface. Light incident on the front surface passes through the clearthermoplastic layer, impinges on the rear retroreflecting surface, andis reflected back out through the front surface in a predetermineddirection (e.g., aligned with and/or parallel to the direction ofincidence). In this manner, incident light can be used to illuminatemarkings, words, and other information in an otherwise dark environment.

The retroreflecting surface can be formed by a repeating array ofretroreflective elements embossed in the thermoplastic layer. Theretroreflective elements can comprise, for example, corner-cube elementswhich each have three flat faces arranged mutually at right angles andconnected by edges which join at an apex. (See e.g., U.S. Pat. No.1,906,655, U.S. Pat. No. 3,332,327, U.S. Pat. No. 3,541,606, U.S. Pat.No. 3,833,285, U.S. Pat. No. 3,873,184, and/or U.S. Pat. No. 3,923,378.See also, U.S. Pat. No. 6,767,102 which is assigned to the assignee ofthe present invention and the entire disclosure which is herebyincorporated by reference).

Over the years, retroreflective sheet structures have been incorporatedinto a wide range of end products including, for example, vehiclemarkings, highway signs, and construction barrels. In these and otherapplications, extended outdoor durability is important and theretroreflective sheet structure needs to withstand extended sun lightexposure and other harsh environmental conditions. An expected usefullife of twelve years is not considered an unreasonable requirement whena retroreflective structure is being used in a highway situation. Evenin less demanding, more delicate settings, the retroreflective structureis expected to maintain its physical stability and optical reflectivityfor a certain period of time (e.g., one to five years).

Typically, an end product manufacturer will receive a roll ofretroreflective sheeting from an independent supplier. During themanufacture of the end product, the sheeting is unwound from the rolland separated into individual structures for integration into the endproduct. As such, a retroreflective sheeting supplier may be unaware ofwhat end products its sheeting is being used in and/or where the endproduct is being used, especially after an extended period of time.Additionally or alternatively, an end product manufacturer having aplurality of sheeting suppliers (which is dictated by many companies'purchasing policies) may find it difficult to track the identify of thesupplier whose sheeting was used in a particular end product.

SUMMARY OF THE INVENTION

The present invention provides a retroreflective sheet structureincluding indicia that can be used to identify something about thesheeting used to form the structure. For example, the identifyingindicia can allow a sheeting supplier to determine whether a specificretroreflective structure originated from its company and/or an endproduct manufacture to determine what suppliers' sheeting wasincorporated into a particular product. This may be important, forexample, should a retroreflective structure not maintain its physicalstability and/or optical reflectivity for an expected period of time.The present invention allows such a determination, even years after theend product incorporating the reflective sheet structure has been out inthe field.

More particularly, the present invention provides a retroreflectivesheet structure comprising identifying indicia formed on the rearretroreflecting surface of its transparent layer. A supplier can choosean identifying indicia that will not be used by another sheetingsupplier. The identifying indicia is detectable during close inspection(e.g., within 20 cm or less), but does not interfere with theretroreflective qualities of the structure.

The rear retroreflecting surface can comprise a repeating array ofretroreflective elements formed thereon, with some of the elementshaving disturbances arranged in a pattern corresponding to theidentifying indicia. For example, if the retroreflective elements aremicrocubes, a small percentage of the cube faces can have aplanar-disturbance thereon. Forming the identifying indicia in thismanner allows an existing tool plate (having only undisturbedretroreflective elements) to be modified to practice the invention.Specifically, for example, the existing tool plate can be etched (e.g.,laser etched) to create the inverse of the desired planar-disturbances.

These and other features of the invention are fully described andparticularly pointed out in the claims. The following description anddrawings set forth in detail a certain illustrative embodiment of theinvention which is indicative of but one of the various ways in whichthe principles of the invention may be employed.

DRAWINGS

FIGS. 1A, 1B, and 1C are schematic drawings of end products whichincorporate a retroreflective sheet structure according to the presentinvention, the end products being a vehicle, a highway sign, and aconstruction barrel, respectively.

FIGS. 2A and 2B are top and side views, respectively, of theretroreflective sheet structure isolated from the end product.

FIG. 2C is a close-up bottom view of a retroreflective element.

FIG. 2D is a close-up bottom view of a retroreflective element with aplanar disturbance on one of its faces.

FIG. 3 is a perspective view of a roll of retroreflective sheeting whichwould be supplied to an end product manufacturer for fabrication intothe retroreflective sheet structure, the sheeting including an embossedthermoplastic layer.

FIGS. 4A and 4B are schematic views of a method of embossing thethermoplastic layer.

FIG. 4C is a close-up side view of a pyramid projection of the toolingplate used in the method of embossing the thermoplastic layer.

FIGS. 5A and 5B are schematic views of a method of modifying a toolingplate to emboss the thermoplastic layer.

FIG. 5C is a close-up side view of a pyramid portion of the toolingprior to laser etching.

FIG. 5D is a close-up side view of a pyramid portion of the toolingafter laser etching.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, and initially to FIGS. 1A, 1B, and 1C, aretroreflective sheet structure 10 according to the present invention isshown incorporated into an end product 12. In the illustratedembodiments, the end products 12 are a vehicle, a highway sign, and aconstruction barrel, respectively. With these end products 12, extendedoutdoor durability is important and the sheet structure 10 needs towithstand extended sun light exposure and other harsh environmentalconditions for an expected period of time. However, the presentinvention is not limited to outdoor situations, and the end product 12can be any product which incorporates the retroreflective sheetstructure 10 according to the present invention. In any event, the endproduct 12 includes a mounting surface 14 to which the sheet structure10 is attached and, preferably, adhesively attached.

Referring now to FIGS. 2A and 2B, the retroreflective sheet structure 10is shown isolated from the end product 12. The structure 10 comprises atransparent layer 20, a reflection-aiding layer 22, a backing layer 24,and an adhesive layer 26. A removable release layer 28 (shown inphantom) can be provided to cover the adhesive layer 26 duringpre-mounting stages of end product fabrication.

The transparent layer 20 can comprise any suitable thermoplasticmaterial which is compatible with desired manufacturing methods (e.g.,acrylic, vinyl, polymethylacrylate, polycarbonate, polyurethane,polysulfone, polyarylate, polyether imide, polyetherimide,cyclo-olefinic copolymer, and/or acrylonitrile butadiene styrene). Thereflection-aiding layer 22 can be a metallized film, granular silicaparticles, or any other acceptably reflective material. The backinglayer 24 can serve as a space-filler behind the layers 20/22 and/or as acarrier for the adhesive layer 26 and, to this end, can comprise apaper, plastic, metal, or other sheet/substrate which performs thesefunctions. The adhesive layer 26 is used to attach the reflective sheetstructure 10 to the mounting surface 14 of the end product 12 and cancomprise a pressure-sensitive or heat-activated adhesive.

The transparent layer 20 has a front light-receiving surface 30 and arear retroreflective surface 32 on which a repeating array ofretroreflective elements 34 are formed. Light incident on the smoothfront surface 30 passes through the clear thermoplastic layer 20,impinges on the retroreflective elements 34, and is reflected back outthrough the front surface 30 in a predetermined direction (e.g., alignedwith and/or parallel to the direction of light incidence).

As is best seen in FIG. 2C, the retroreflective elements 34 arepreferably microcubes which each comprise a three flat faces 36 arrangedmutually at right angles and connected by edges 38 that meet an apex 40.The size, shape and arrangement of the faces 36 determines, and can bevaried to adjust, optical qualities. The cube area of eachretroreflective element 34 (i.e., the area enclosed by the cube shapedefined by pyramid of the perimeter of the three faces 36 in thedirection of the principle refracted ray) can be about 1 mm.sup.2 orless.

In a large majority of the retroreflective elements 34 (e.g., more than80%, more than 90%, more than 95%, and/or more than 98%), the threefaces 36 are planar without any outthrusts or depressions. (See FIG.2D.) However, in a selected few of the retroreflective elements 34, oneface 36 includes a planar-disturbance 42 which disrupts the planarprofile of the face 36. (See FIG. 2E.) The retroreflective elements 34having a planar-disturbance 42 are arranged in a pattern correspondingto an identifying indicia 44. For example, in the illustratedembodiment, the planar disturbances 42 collectively form an “ABC” logowhich could correspond to the supplier of a retroreflective sheeting(namely retroreflective sheeting 48, introduced below) used to fabricatethe structure 10. It is expected that a supplier will choose theidentifying indicia 44 so that will not be another sheeting supplier.

In the illustrated embodiment, the planar-disturbance 42 is a protrusionand, more particularly, a protuberance having a knob-like shape.However, other protrusion geometries are certainly possible with, andcontemplated by, the present invention. Moreover, other disturbances inthe planar profile of the selected faces 36 could be used instead of, orin addition to, the protuberances 42. For example, indentations,notches, pits, depressions or other recesses could be used to form theidentifying indicia 44. The planar-disturbances 42 can be same, similar,or different among the “disturbed” retroreflective elements 34.

The planar-disturbance 42 will occupy only a small percentage (e.g.,less than 30%, less than 20%, less than 10%, and/or less than 5%) of thesurface area of the disturbed face 36 of the respective retroreflectiveelement 24. Thus, most the reflective regions of the disturbed faces 36are left intact. Moreover, the identifying indicia 44 collectivelyformed by the disturbances 42 will preferably occupy an area of lessthan 16 cm.sup.2 on the surface 32 of the thermoplastic layer 20. Inthis manner, the identifying indicia 44 will be detectable during closeinspection (i.e., within 20 cm or less of the structure 10), but willnot interfere with, or detract from, the retroreflective qualities ofthe structure 10. That being said, larger identifying indicia 44 couldbe used if such interference and/or detraction is acceptable or desiredin a particular situation.

Referring now to FIG. 3, retroreflective sheeting 48 is shown which canbe used to create a plurality of the structures 10. Typically, amanufacturer of the end product 12 (or of subassemblies therefor) willreceive a roll of the retroreflective sheeting 48 from an independentsupplier. During the manufacture of the end product 12, the sheeting 48is unwound from the roll and separated into individual structures 10 forintegration into the end product 12. To this end, the sheeting 48 cancomprise a transparent thermoplastic layer 50, a reflection-aiding layer52, a backing layer 54, an adhesive layer 56, and a removable releaselayer 58. The transparent layer 50 includes the repeating array ofretroreflective elements 34 and includes the identifying indicia 44 atpredetermined positions and/or intervals to insure that each structure10 includes at least one such indicia 44.

Referring now to FIGS. 4A and 4B, a method of making the thermoplasticlayer 50 is schematically shown. In this method, a thermoplastic film 60is embossed by a tool plate 62, and then cooled to solidify the embossedmicrostructure. The tooling plate 62 used in this method can be avariety of sizes with widths/lengths ranging from, for example, fiveinches to sixty inches. For example, the tooling plate 62 can be thirtyinches wide and sixty inches long, or it can be five inches wide andfive inches long.

As is best seen by referring to FIG. 4C, the tool plate 62 has atopography corresponding to the inverse of the retroreflective elements34. Specifically, the tool plate 62 has a series of pyramid (or otherpolyhedron) projections 64 comprising three faces 66 arranged atmutually right angles and connected by edges 68 which join at an apex70. The majority of the faces 66 of the tool plate 62 have flat planarprofiles to produce retroreflective elements 34 with three planar faces36 without any out-thrusts or depressions. However, the faces 66′ of thetool plate 62 intended to collectively form the identifying indicia 44include a disturbance 72 corresponding to the inverse of the intendedplanar-disturbance 42. Thus, if the intended geometry of the disturbance42 is a protrusion, as shown, the disturbance 72 will be a depression.(Likewise, if the intended geometry of the disturbance 42 was a recess,the disturbance 72 would be a projection.)

Referring now to FIG. 5, a method of making the tooling plate 62 isschematically shown. In this method, a tooling plate 92 is provided inwhich has a topography corresponding to the retroreflective elements 34all having three flat planar faces 36. (FIG. 5A.) Specifically, forexample, the tooling plate 92 could comprise a series of pyramidprojections 64 comprising three flat face sections 66 arranged atmutually right angles and connected by edges 68 which all join at anapex 70. After identifying indicia 44 has been chosen, and the locationof the indicia 44 relative to the overall thermoplastic layer isdetermined, the face sections 66′ that correspond to the location of theplanar-disturbances 72 can be charted.

The planar-disturbance 72 is then formed on each charted face section66′ by a suitable technique such as the application of energy,chemicals, or machining. For example, a laser beam (e.g., a YAG pulselaser or a CO.sub.2 laser) can be focused on the face section 66′ tomelt the surface and form the disturbance 72. Chemicals can beparticularly useful with a plastic material, in that the application ofa drop of solvent on the charted face section can cause the surface topucker to form the disturbance 72. Machining methods can include, forexample, micro-drilling.

Advantageously, the present invention does not require the constructionof a new tooling plate 62, but rather allows the retrofitting anexisting plate 92.

One may now appreciate that the present invention provides aretroreflective sheet structure 10 wherein the identifying indicia 44allows a sheeting supplier to determine whether a particular structureoriginated from its company and/or an end product manufacture todetermine what suppliers' sheeting was incorporated into a particularend product. Although the invention has been shown and described withrespect to certain preferred embodiments, it is obvious that equivalentand obvious alterations and modifications will occur to others skilledin the art upon the reading and understanding of this specification. Thepresent invention includes all such alterations and modifications and islimited only by the scope of the following claims.

What is claimed is:
 1. A method of making a transparent layer (50) thathas a front-light receiving surface (30) and a rear retroreflectivesurface (32) that includes a repeating array of retroreflective elements(34) and identifying indicia (44) at predetermined positions and/orintervals for retroreflective sheeting (48), said method comprising thesteps of: embossing a thermoplastic film (60) with a tool plate (62);and cooling the embossed thermoplastic film (60); wherein the tool plate(62) has a topography corresponding to the inverse of reflectiveelements (34) and identifying indicia (44).
 2. The method of claim 1wherein the tool plate has a length in the range of about 5 inches to 60inches.
 3. The method of claim 1 wherein the tool plate has a width inthe range of about 5 inches to 60 inches.
 4. The method of claim 1wherein the majority of retroreflective elements (34) are planar withoutany outthrusts or depressions.
 5. The method of claim 1 wherein one face(36) of at least some of the retroreflective elements (34) have planardisturbances (42).
 6. The method of claim 5 wherein the planardisturbances (42) correspond to the identifying indicia (44).
 7. Themethod of claim 1 wherein the topography of the tool plate (62)comprises a series of polyhedron projections (64).
 8. The method ofclaim 7 wherein the polyhedron projections are pyramids each havingthree faces (66) arranged at mutually right angles and connected byedges (68) which join at an apex (70).
 9. The method of claim 8 whereinthe majority of faces (66) have flat planar surfaces.
 10. The method ofclaim 9 wherein at least some of the remaining faces (66) have adisturbance formed thereon.
 11. The method of claim 10 wherein thedisturbances (72) corresponding to the inverse of the intended planardisturbances (42).
 12. The method of claim 10 wherein the disturbance(72) is a depression.
 13. The method of claim 10 wherein the disturbance(72) is a projection.
 14. The method of claim 5 wherein most of thedisturbed face (36) reflective regions remain intact.
 15. The method ofclaim 1 wherein the identifying indicia (44) becomes visually detectableat an inspection distance of about 20 cm or less.
 16. The method ofclaim 1 wherein the identifying indicia (44) will not interfere with ordetract from the retroreflective qualities of the structure.